One type of antenna is the monopole antenna which has a length corresponding to a quarter wavelength of its design frequency. This design is efficient, robust, provides a good bandwidth and, typically, can be a good match to a 50 .OMEGA. input impedance. FIG. 1 shows an example of such an antenna. Nevertheless the quarter wavelength monopole is relatively long and is limited to multiple frequency usage at the third and fifth harmonic frequencies. Whilst it may be possible to operate the antenna in two frequency bands associated with different radio systems, where the operating frequency of one band is a harmonic of the other operating frequency band, such an overlap would not be tenable because of the inevitable interference effects. Further, radio frequency spectrum allocation is not, typically, based upon the harmonics of a primary band.
A second type of antenna is the so-called top loaded monopole which is similar in many respects to the first type of antenna as described above but is three dimensional and has a circular planar element attached to the top of the monopole. FIG. 2 shows an example of such an antenna configuration. This design can only operate in a single frequency band, has increased lateral dimensions and does not have a high efficiency--nevertheless, such an antenna has found application in many applications, where a reduction in overall dimensions is preferred.